Manipulating molecular aggregation and crystalline behavior of A-DA'D-A type acceptors by side chain engineering in organic solar cells

Alkyl chains engineering plays an important role in photovoltaic materials for organic solar cells. Herein, three A-DA'D-A (acceptor-donor-acceptor'-donoracceptor) type acceptors named Y6, Y6-C4, and Y6-C5 with different branching position on the pyrrole motif are discussed and the relationship between molecular aggregation, crystalline, and device performance are systematically investigated. The distance between the branching position and the main backbone affects their optical absorption and energy levels. Y6-C4 and Y6-C5 with the branching position at the fourth and fifth carbon of the alkyl chain show blue-shifted absorption and increased electrochemical bandgaps, compared with Y6 with the branching position at the second carbon of the alkyl side chain. In addition, this distance influences the molecular aggregation and crystalline behavior of the donor/acceptor blends. Compared with Y6-C4, Y6-C5 possesses a stronger crystalline and aggregate ability in the blends with a lower non-radiative energy loss, which results in a higher open circuit voltage (Voc) of 0.88 V. Finally, Y6-C5-based binary device achieved a high power conversion efficiency up to 16.73% with afill factor (FF) of 0.78. These results demonstrate that the side chain engineering is an effective strategy for tuning the molecular aggregation and crystalline to improve photovoltaic performance of the A-DA'D-A type acceptors.